1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus and a method of acquiring ultrasonic images for generating a Doppler spectrum image. Specifically, it relates to an ultrasonic imaging apparatus and a method of acquiring ultrasonic images that automatically adjust the velocity range of a Doppler spectrum image.
2. Description of the Related Art
Conventionally known ultrasonic imaging apparatuses concomitantly adopt an ultrasonic pulse reflection method and an ultrasonic Doppler method, in order to obtain cross-sectional images of diagnostic sites and the bloodstream information thereof through ultrasonic manipulation using one ultrasonic probe to display at least the bloodstream information in real time. These ultrasonic imaging apparatuses are used for analyzing the Doppler shift frequency based on the principle of the ultrasonic Doppler method, to obtain bloodstream information from the results of the analysis, the principle of the ultrasonic Doppler method meaning that the received frequency shifts slightly from the transmitted frequency due to the Doppler effect of the ultrasound transmitted to and received at a diagnostic site having a flow such as that of blood in a body, so that the shift frequency (Doppler shift frequency) is proportional to the blood velocity.
In the above-mentioned ultrasonic imaging apparatuses, items (parameters) to be used for a diagnosis are measured for a spectrum image of a Doppler frequency that displays the results of a frequency analysis with the Fast Fourier Transform (FFT) of an obtained Doppler signal in spectrum display with the frequency as the vertical axis, time t as the a horizontal axis, and power (strength) of frequency components as luminance (tone).
The operational flow of this measurement process is described in sequence. (1) On a spectrum image of a Doppler frequency, positions of a maximum flow velocity Vp (V peak) that corresponds to the maximum frequency and a mean velocity Vm (V mean) that corresponds to the mean frequency within a frequency distribution in the axial direction of the frequency are calculated.
(2) Each change in time of the maximum flow velocity Vp and mean flow velocity Vm is traced in the axial direction of the time. (3) On a trace waveform that shows curves of temporal positional changes of the Vp and Vm, a systolic waveform peak PS (Peak of Systolic) and a diastolic waveform peak ED (End of Diastolic) are simultaneously detected during each cardiac cycle (1 heartbeat). (4) Based on information of the PS and ED, various parameters (indexes) for a diagnosis such as an intravascular blood flow volume, HR (Heart Rate) of pulsatile flow, PI (Pulsatility Index), and RI (Resistance Index), etc. are measured, and a process to display those measurements (parameter measurement process) is conducted.
The above-mentioned trace waveform detection processes for Vp and Vm, peak detection processes for PS and ED, and parameter measurement processes such as PI and RI, etc. are basically conducted through manual operation using a freeze image. Moreover, in recent years, ultrasonic imaging apparatuses that conduct the same processes with automatic operation using real-time images have also been widely used.
In the Pulse Doppler (PW) method, a pulse with a predetermined repetition frequency is transmitted and the frequency of the received signal is analyzed with a predetermined sampling frequency. When the sampling frequency fs for this frequency analysis is lower than the Doppler shift frequency, aliasing (folding) occurs. Therefore, to prevent this, it is necessary to increase the pulse repetition frequency (PRF) and shorten the intervals between each observation time. In this case, designating a location to be measured consequently decides the maximum PRF, and once the PRF is decided, the measurable maximum blood velocity is also decided.
This measurable maximum blood velocity is called the velocity range.
For example, to measure the velocity of blood flow that is approximately 30 cm/s, if a velocity range of approximately 10 cm is set, aliasing occurs and the blood flow cannot be measured. Thus, in this case, it is necessary to set the velocity range at approximately 50 cm/s.
With a Doppler spectrum display, when the velocity range is too small, a folding portion is generated as described above. In such a case, an operator can manually set the Doppler velocity range at a higher value, by which the folding portion falls within the Nyquist rate (half of the PRF) and a Doppler spectrum image that is smooth on the display can be obtained.
In contrast, when the velocity range is too large, the waveform of a spectrum becomes small, causing difficulty in observation. In such a case, an operator can obtain a Doppler spectrum image that efficiently uses the top and bottom portions of the display screen and is easy to observe by setting the velocity range to a low value.
Moreover, in the ultrasonic Doppler method, a positive sign is assigned to blood flow that goes toward the ultrasonic probe in the direction of blood flow. Moreover, a negative sign is assigned to blood flow that goes away from the probe. When an ultrasonic probe is applied to a specific vessel and the vessel is an artery, the velocity of the blood flow changes depending on heartbeat but does not change between positive and negative, usually placing a disproportionate emphasis on either positive or negative.
For example, with Doppler spectrum display, when a folding portion occurs, an operator may shift the baseline (BL=0) of the Doppler spectrum image by manipulating the baseline shift switch. This is called an adjustment of velocity offset. By shifting this baseline by only −0.25 (amount of baseline shift=−0.25), the folding portion moves beyond the Nyquist rate and a Doppler spectrum that is smooth on the display can be obtained.
A Doppler spectrum image that is obtained with an ultrasonic imaging apparatus will now be described with reference to FIG. 1. FIG. 1 is a diagram that shows cross-sectional images and Doppler spectrum images acquired by means of an ultrasonic imaging apparatus. A case in which cross-sectional images and Doppler spectrum images are acquired and displayed with a carotid artery as the diagnostic site will now be described.
For example, in a screen 110, when a vessel shown in an image in which a B-mode cross-sectional image 100 and a color Doppler image 101 are superimposed is designated by a range gate 102, which is used to designate the location at which a Doppler spectrum image is acquired, a Doppler spectrum image that shows the time change of the blood velocity distribution at that location is obtained and displayed on the screen. On a screen 111, a Doppler spectrum image 103 with a PRF for determining the velocity range (measurable maximum blood velocity) of 7.1 (kHz) and a velocity offset (BLS: Baseline Shift) of 0 (Hz) is shown (part indicated with a dotted line in the screen 110).
Furthermore, for measurement of the state of blood flow based on the shape of the peak determined through an auto trace of the Doppler spectrum image 103, an operator adjusts the pulse repetition frequency (PRF) and velocity offset (BLS) in a screen 120 so that the state of the blood flow is displayed with a specific ratio in the center of the velocity range (vertical axis). For example, by changing the PRF to 5 (kHz) and shifting the BLS to the negative side, the Doppler spectrum image 103 is enlarged and displayed as shown in screen 121 (part indicated with a dotted line in the screen 120).
When measuring the blood velocity, etc. with an ultrasonic imaging apparatus, the blood velocity that is measured changes largely depending on any disorders and the physical condition of the subject, how the probe is applied (angle), the location and width of the intravascular range gate with a PW Doppler, and the diagnostic site. Therefore, conventionally, an operator has performed optimization each time by adjusting the velocity range of the apparatus and shifting the baseline to measure HR, PI, and RI from an enlarged waveform. However, it is cumbersome to adjust the PRF and velocity offset (BLS) to correspond to the velocity range each time the state of blood flow to be diagnosed changes.
Therefore, ultrasonic imaging apparatuses that provide automatic operation for adjustments of the velocity range and velocity offset of a Doppler spectrum image and improved operability of blood flow measurement so that an operator does not need to pay attention to the setup of the apparatus have been proposed (for example, Japanese published unexamined application No. 2005-185731). According to a technique related to the conventional art, a histogram that shows the frequency distribution of a velocity is created by calculating the frequency of a maximum velocity (frequency) based on a Doppler waveform acquired at a predetermined timing (for example, 1 heartbeat), and based on the histogram, the velocity range is determined so that the Doppler waveform is displayed within α% (for example, 70%) of the vertical direction of the display area, and feedback is given.
At this time, to automatically adjust the velocity range of a Doppler spectrum image, stability and reliability during the measurement of a Doppler spectrum are important issues. However, the circulatory organs (heart) have valves to circulate blood, etc., and therefore signals (hereinafter referred to as “valve signals”) are generated when the valves operate. Therefore, with a conventional ultrasonic imaging apparatus, it was difficult to automatically adjust the velocity range for only a blood flow component, particularly for Doppler blood flow diagnoses of a circulatory organ (heart), because valve signals with high power are incorporated along with blood flow signals.